As usual, \(C_F=(N_C^2-1)/(2 N_C)\)  is a color factor and \(e_a (e_b)\)  is the charge of the flavor-\(a (b)\)  quark
in units of the positron charge. The summation over the same flavor labels is understood.
The twist-2 contribution only affects the subprocess amplitudes for quark helicity non-flip, the
quark helicity-flip amplitudes are zero. Leaving aside the pion DA the subprocess
amplitudes are expressed in terms of the Lorentz invariant Mandelstam variables. Thus, in any other c.m.s.
the expressions (REF ) and (REF ) hold too. In any case we assume that in a c.m.s. in
which factorization holds, the pion DA is the usual one. For the wide-angle case this is the symmetric
frame (REF ) but for the DVMP limit, \(t\rightarrow 0\) , which we occasionally discuss, this is Ji's
frame [1]}.
